Porterville
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Porterville clear sky chart from 2005-11-06 to 2025-03-27.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	5% 3 1 3 2 2 3 2 4 5% 69%	112874
	Night	7% 4 1 3 2 2 2 2 4 6% 66%	115286
Transparency	Day	19% 3 8% 19% 26% 25%	112626
	Night	22% 3 9% 21% 25% 21%	115125
Seeing	Day	7% 2 14% 33% 31% 13%	15814
	Night	9% 2 7% 38% 37% 7%	25534

By Month

Percentage Cloud Cover SW (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	8% 8% 3 6% 4 4 4 3 6% 7% 46%	3639
	11th to 20th	Day	6% 4 2 6% 4 3 3 3 5% 6% 56%	3936
	21st to end	Day	11% 7% 2 6% 4 4 4 3 6% 8% 45%	4510
February	1st to 10th	Day	13% 7% 2 5% 3 3 3 3 6% 6% 50%	1993
	11th to 20th	Day	10% 7% 2 4 3 3 4 3 6% 8% 50%	1950
	21st to end	Day	13% 8% 2 6% 3 3 4 3 5% 6% 47%	1650
March	1st to 10th	Day	13% 8% 3 7% 4 4 4 4 6% 9% 40%	3238
	11th to 20th	Day	6% 4 1 4 3 3 3 4 5% 7% 58%	3457
	21st to end	Day	12% 6% 2 5% 4 4 5% 5% 6% 8% 43%	4206
April	1st to 10th	Day	8% 4 1 4 3 3 3 3 5% 6% 60%	2280
	11th to 20th	Day	7% 4 1 3 3 3 3 3 4 7% 61%	2249
	21st to end	Day	7% 2 1 3 2 2 3 3 5% 6% 66%	2280
May	1st to 10th	Day	2 2 1 2 2 2 2 2 3 4 78%	3912
	11th to 20th	Day	2 1 1 1 2 2 2 3 5% 80%	4071
	21st to end	Day	3 1 1 1 1 2 1 3 3 83%	4630
June	1st to 10th	Day	2 2 1 2 2 2 2 2 4 6% 75%	2660
	11th to 20th	Day	1 1 1 1 1 1 3 3 88%	2660
	21st to end	Day	1 1 1 1 1 1 1 2 3 87%	2660
July	1st to 10th	Day	1 1 1 1 2 2 92%	4420
	11th to 20th	Day	1 1 1 1 1 2 2 2 3 85%	4420
	21st to end	Day	1 1 1 1 1 1 1 2 3 88%	4485
August	1st to 10th	Day	1 1 1 1 1 1 1 2 3 87%	3783
	11th to 20th	Day	1 1 1 1 1 1 1 2 92%	3724
	21st to end	Day	1 1 1 1 1 2 2 91%	4180
September	1st to 10th	Day	1 1 1 1 1 1 2 2 89%	2280
	11th to 20th	Day	1 1 1 2 1 1 2 3 86%	2195
	21st to end	Day	1 1 1 1 1 2 2 3 4 84%	2090
October	1st to 10th	Day	2 1 1 1 1 2 2 4 5% 79%	3407
	11th to 20th	Day	2 2 1 2 2 2 3 2 3 4 78%	3161
	21st to end	Day	4 2 1 3 2 2 3 3 5% 7% 67%	3396
November	1st to 10th	Day	5% 5% 1 4 2 3 3 3 4 8% 62%	1764
	11th to 20th	Day	6% 5% 2 4 3 3 3 3 3 7% 61%	1800
	21st to end	Day	9% 6% 2 5% 3 3 4 3 4 6% 54%	1800
December	1st to 10th	Day	6% 4 2 5% 4 4 4 3 5% 7% 57%	2950
	11th to 20th	Day	12% 6% 2 7% 5% 4 4 5% 6% 8% 40%	3034
	21st to end	Day	13% 12% 4 9% 4 4 5% 4 6% 8% 30%	4004
January	1st to 10th	Night	12% 8% 3 5% 3 3 3 2 5% 7% 50%	6306
	11th to 20th	Night	8% 5% 2 6% 3 3 3 2 4 6% 59%	6034
	21st to end	Night	14% 7% 2 5% 3 3 3 3 4 7% 48%	6468
February	1st to 10th	Night	14% 6% 2 5% 4 3 3 3 4 7% 50%	2404
	11th to 20th	Night	13% 4 1 3 2 2 2 3 5% 8% 56%	2338
	21st to end	Night	14% 6% 1 4 3 3 3 3 3 5% 55%	1980
March	1st to 10th	Night	14% 6% 2 5% 4 4 3 3 6% 9% 44%	3856
	11th to 20th	Night	7% 4 2 3 3 3 4 3 5% 7% 59%	3738
	21st to end	Night	14% 7% 3 6% 4 4 4 3 5% 6% 44%	3584
April	1st to 10th	Night	9% 5% 2 4 2 3 2 3 4 7% 59%	1900
	11th to 20th	Night	7% 3 1 3 3 3 2 3 4 6% 65%	1879
	21st to end	Night	7% 3 1 2 2 1 2 2 3 5% 70%	1900
May	1st to 10th	Night	7% 2 1 2 1 1 2 1 2 4 78%	3062
	11th to 20th	Night	3 1 1 1 1 2 2 1 4 6% 78%	2834
	21st to end	Night	3 2 1 2 1 1 1 1 2 3 84%	2933
June	1st to 10th	Night	3 2 2 2 2 2 2 3 6% 76%	1520
	11th to 20th	Night	1 1 1 2 1 1 1 2 3 87%	1520
	21st to end	Night	1 1 1 1 1 3 2 90%	1520
July	1st to 10th	Night	1 1 2 2 92%	2540
	11th to 20th	Night	1 1 1 1 1 1 1 2 4 86%	2540
	21st to end	Night	1 2 2 1 1 1 1 3 88%	3056
August	1st to 10th	Night	1 1 2 1 1 1 1 2 4 85%	3166
	11th to 20th	Night	1 1 1 1 2 93%	3126
	21st to end	Night	1 1 1 1 1 2 93%	3498
September	1st to 10th	Night	1 1 1 1 1 1 1 2 90%	1900
	11th to 20th	Night	1 1 1 1 1 1 1 3 3 85%	1984
	21st to end	Night	2 1 1 1 1 2 1 2 4 85%	2090
October	1st to 10th	Night	2 1 1 1 1 1 1 2 3 85%	3569
	11th to 20th	Night	2 1 2 1 1 2 2 4 5% 80%	3777
	21st to end	Night	4 3 1 2 1 2 2 1 3 5% 77%	4171
November	1st to 10th	Night	7% 3 1 3 3 2 2 3 4 6% 66%	2535
	11th to 20th	Night	6% 3 1 4 2 2 3 2 4 7% 66%	2600
	21st to end	Night	10% 6% 1 4 3 2 3 3 5% 7% 57%	2600
December	1st to 10th	Night	5% 4 1 5% 3 3 3 3 5% 6% 61%	4240
	11th to 20th	Night	11% 6% 2 7% 4 4 4 4 7% 10% 43%	5166
	21st to end	Night	10% 9% 3 8% 5% 5% 5% 4 7% 9% 35%	6952

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	34% 6% 20% 21% 16% 2	3639
	11th to 20th	Day	28% 5% 13% 15% 21% 19%	3936
	21st to end	Day	36% 5% 12% 15% 25% 7%	4510
February	1st to 10th	Day	34% 4 10% 26% 19% 6%	1993
	11th to 20th	Day	32% 4 12% 27% 19% 7%	1960
	21st to end	Day	39% 3 11% 28% 16% 3	1650
March	1st to 10th	Day	40% 5% 14% 22% 16% 3	3238
	11th to 20th	Day	24% 4 15% 36% 16% 6%	3457
	21st to end	Day	36% 6% 14% 27% 14% 3	4206
April	1st to 10th	Day	26% 3 13% 30% 21% 7%	2280
	11th to 20th	Day	23% 3 12% 30% 23% 9%	2252
	21st to end	Day	20% 3 10% 33% 27% 6%	2280
May	1st to 10th	Day	12% 2 9% 27% 36% 15%	3912
	11th to 20th	Day	9% 2 6% 27% 40% 16%	4074
	21st to end	Day	9% 2 5% 28% 41% 15%	4630
June	1st to 10th	Day	13% 2 9% 23% 34% 20%	2660
	11th to 20th	Day	5% 1 5% 14% 39% 35%	2660
	21st to end	Day	6% 1 2 8% 31% 52%	2660
July	1st to 10th	Day	3 1 1 5% 14% 76%	4351
	11th to 20th	Day	7% 2 2 6% 18% 65%	4345
	21st to end	Day	6% 1 2 12% 30% 50%	4343
August	1st to 10th	Day	7% 1 2 6% 36% 48%	3770
	11th to 20th	Day	3 1 1 6% 27% 62%	3760
	21st to end	Day	4 1 1 9% 34% 51%	4179
September	1st to 10th	Day	6% 1 2 9% 30% 52%	2280
	11th to 20th	Day	6% 2 2 17% 44% 29%	2195
	21st to end	Day	8% 1 2 17% 40% 31%	2090
October	1st to 10th	Day	9% 1 4 13% 22% 51%	3407
	11th to 20th	Day	12% 2 5% 24% 35% 22%	3161
	21st to end	Day	17% 2 5% 18% 35% 24%	3396
November	1st to 10th	Day	21% 3 9% 24% 29% 15%	1764
	11th to 20th	Day	25% 3 10% 25% 28% 9%	1800
	21st to end	Day	32% 3 7% 22% 23% 13%	1800
December	1st to 10th	Day	26% 4 11% 29% 21% 8%	2950
	11th to 20th	Day	38% 6% 13% 20% 18% 5%	3034
	21st to end	Day	47% 9% 14% 22% 7% 2	4004
January	1st to 10th	Night	34% 4 18% 26% 16% 1	6306
	11th to 20th	Night	28% 3 15% 14% 27% 12%	6034
	21st to end	Night	36% 4 12% 20% 21% 6%	6468
February	1st to 10th	Night	37% 2 8% 28% 18% 7%	2404
	11th to 20th	Night	27% 4 13% 27% 22% 7%	2339
	21st to end	Night	34% 3 12% 30% 17% 4	1980
March	1st to 10th	Night	36% 5% 13% 25% 18% 3	3856
	11th to 20th	Night	24% 5% 15% 36% 14% 6%	3738
	21st to end	Night	41% 4 15% 24% 13% 2	3584
April	1st to 10th	Night	27% 3 13% 30% 20% 8%	1900
	11th to 20th	Night	22% 3 12% 30% 25% 9%	1879
	21st to end	Night	18% 2 10% 34% 31% 4	1900
May	1st to 10th	Night	15% 1 5% 26% 38% 16%	3062
	11th to 20th	Night	9% 2 4 26% 42% 18%	2835
	21st to end	Night	9% 2 5% 24% 43% 17%	2933
June	1st to 10th	Night	13% 2 8% 22% 35% 20%	1520
	11th to 20th	Night	7% 1 4 15% 37% 37%	1520
	21st to end	Night	4 1 2 8% 34% 52%	1520
July	1st to 10th	Night	3 1 5% 14% 78%	2500
	11th to 20th	Night	6% 1 2 6% 15% 70%	2492
	21st to end	Night	6% 2 2 10% 26% 55%	2961
August	1st to 10th	Night	7% 1 1 8% 33% 49%	3156
	11th to 20th	Night	2 1 1 7% 27% 62%	3156
	21st to end	Night	3 1 1 9% 30% 57%	3498
September	1st to 10th	Night	5% 1 1 10% 29% 54%	1900
	11th to 20th	Night	6% 2 3 16% 46% 27%	1984
	21st to end	Night	7% 1 2 18% 39% 32%	2090
October	1st to 10th	Night	8% 1 4 11% 25% 51%	3569
	11th to 20th	Night	10% 1 4 24% 41% 21%	3777
	21st to end	Night	14% 2 3 20% 36% 24%	4171
November	1st to 10th	Night	21% 3 9% 25% 26% 16%	2535
	11th to 20th	Night	20% 3 13% 24% 29% 12%	2600
	21st to end	Night	29% 2 8% 22% 29% 10%	2600
December	1st to 10th	Night	24% 3 14% 32% 20% 7%	4240
	11th to 20th	Night	35% 6% 15% 23% 18% 3	5166
	21st to end	Night	43% 7% 18% 24% 7% 2	6952

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	20% 2 13% 38% 22% 7%	200
	11th to 20th	Day	13% 1 12% 40% 23% 10%	327
	21st to end	Day	13% 3 11% 35% 28% 10%	440
February	1st to 10th	Day	16% 2 15% 34% 24% 10%	497
	11th to 20th	Day	14% 4 18% 36% 21% 7%	495
	21st to end	Day	14% 2 18% 40% 23% 4	414
March	1st to 10th	Day	15% 5% 22% 34% 21% 4	500
	11th to 20th	Day	14% 3 20% 35% 23% 6%	500
	21st to end	Day	12% 5% 22% 38% 17% 5%	531
April	1st to 10th	Day	10% 6% 21% 33% 22% 9%	460
	11th to 20th	Day	10% 7% 23% 30% 23% 7%	452
	21st to end	Day	5% 7% 23% 30% 24% 12%	460
May	1st to 10th	Day	4 5% 25% 35% 24% 8%	463
	11th to 20th	Day	5% 2 20% 33% 26% 13%	488
	21st to end	Day	7% 4 17% 30% 32% 11%	529
June	1st to 10th	Day	2 3 15% 30% 37% 14%	490
	11th to 20th	Day	1 2 15% 31% 38% 12%	490
	21st to end	Day	1 1 12% 34% 39% 13%	490
July	1st to 10th	Day	1 6% 29% 45% 19%	486
	11th to 20th	Day	1 1 3 21% 48% 26%	484
	21st to end	Day	2 1 2 27% 45% 24%	531
August	1st to 10th	Day	2 1 9% 34% 36% 18%	487
	11th to 20th	Day	1 7% 33% 40% 18%	477
	21st to end	Day	1 6% 30% 43% 20%	539
September	1st to 10th	Day	2 1 7% 35% 36% 19%	490
	11th to 20th	Day	1 1 13% 41% 34% 9%	490
	21st to end	Day	2 2 14% 31% 37% 15%	490
October	1st to 10th	Day	3 2 16% 33% 30% 15%	490
	11th to 20th	Day	3 3 15% 33% 34% 12%	490
	21st to end	Day	3 3 12% 33% 37% 12%	511
November	1st to 10th	Day	8% 12% 29% 36% 15%	300
	11th to 20th	Day	8% 1 11% 34% 29% 17%	300
	21st to end	Day	14% 1 13% 29% 30% 13%	300
December	1st to 10th	Day	15% 3 10% 34% 28% 10%	300
	11th to 20th	Day	15% 2 8% 33% 26% 16%	203
	21st to end	Day	21% 2 11% 35% 22% 7%	220
January	1st to 10th	Night	18% 3 7% 39% 27% 7%	800
	11th to 20th	Night	11% 1 8% 44% 30% 6%	800
	21st to end	Night	17% 1 5% 44% 27% 5%	880
February	1st to 10th	Night	16% 3 7% 41% 27% 6%	800
	11th to 20th	Night	15% 3 9% 40% 27% 7%	784
	21st to end	Night	18% 2 8% 46% 23% 4	660
March	1st to 10th	Night	14% 2 12% 46% 24% 3	800
	11th to 20th	Night	13% 2 9% 41% 29% 6%	800
	21st to end	Night	16% 3 11% 41% 27% 2	862
April	1st to 10th	Night	11% 3 12% 43% 28% 3	760
	11th to 20th	Night	9% 4 13% 41% 31% 3	751
	21st to end	Night	9% 3 13% 36% 34% 5%	760
May	1st to 10th	Night	7% 2 15% 41% 32% 3	694
	11th to 20th	Night	6% 2 14% 36% 39% 3	561
	21st to end	Night	7% 5% 13% 43% 30% 3	518
June	1st to 10th	Night	4 3 9% 43% 37% 4	380
	11th to 20th	Night	2 1 7% 52% 35% 4	380
	21st to end	Night	1 6% 44% 46% 4	380
July	1st to 10th	Night	1 2 32% 60% 5%	370
	11th to 20th	Night	1 1 25% 66% 7%	368
	21st to end	Night	1 1 23% 63% 12%	586
August	1st to 10th	Night	2 3 33% 56% 6%	756
	11th to 20th	Night	1 2 29% 59% 10%	752
	21st to end	Night	1 2 21% 64% 12%	836
September	1st to 10th	Night	2 2 26% 58% 12%	760
	11th to 20th	Night	2 4 38% 49% 7%	760
	21st to end	Night	2 5% 34% 48% 11%	760
October	1st to 10th	Night	4 1 5% 40% 39% 11%	760
	11th to 20th	Night	4 1 5% 37% 43% 10%	760
	21st to end	Night	4 1 6% 38% 37% 14%	836
November	1st to 10th	Night	9% 1 6% 36% 37% 11%	780
	11th to 20th	Night	8% 1 5% 43% 34% 9%	800
	21st to end	Night	13% 2 7% 39% 32% 8%	800
December	1st to 10th	Night	10% 2 6% 46% 29% 7%	800
	11th to 20th	Night	16% 1 8% 40% 30% 6%	800
	21st to end	Night	18% 1 7% 46% 24% 4	880

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